Grazing Intensity and Spatial Heterogeneity in Bare Soil in a Grazing-Resistant Grassland

We used very large scale aerial (VLSA) photography to quantify spatial patterns in bare soil in the northeastern Colorado shortgrass steppe. Using three pairs of pastures stocked at moderate (0.6 animal unit months [AUM] . ha(-1)) versus very heavy (1.2 AUM . ha(-1)) rates, we detected greater bare soil under very heavy (mean = 22.5%) versus moderate stocking (mean = 13.5%; P = 0.053) and a lower coefficient of variation across pastures under very heavy (0.48) versus moderate stocking (0.75; P = 0.032). Bare soil exhibited significant positive spatial autocorrelation across distances of 60-120 m under moderate stocking (Moran's I = 0.14), while patchiness at this scale was eliminated under very heavy grazing (1 = 0.05). Across distances of 120-480 m, we observed no spatial autocorrelation with either stocking rate. Spatial autocorrelation was greatest at a separation distance of 2 m (I = 0.48-0.58) but was unaffected by stocking rate at this scale. Thus, very heavy grazing did not increase spatial autocorrelation in bare soil across scales of 2-480 m. Means and variability in the distribution of bare soil were not influenced by ecological site. Bare soil increased primarily at the scale of individual plant clusters through both increases in the density of small (2-20 cm) bare patch intercepts and increases in the frequency of bare patch intercepts of 20-60 cm (rather than <20 cm). Our approach demonstrates the utility of VESA for analyzing interactions between grazing and other landscape features and highlights the importance of spatially explicit sampling across broad scales (pastures) while testing for potential shifts in patchiness of bare soil at the scale of plant interspaces.